Chlorine is commonly used in water treatment processes for a number of reasons. It acts as a disinfectant, can remove ammonia and other nitrogenous organic compounds, it can also control the taste, smell and colour of the water as well as removing slime or algae. However trihalomethanes (THMs) are produced when chlorine reacts with humic acids, fulvic acids, and other Natural Organic Material (NOM) present in the water.
Trihalomethanes
THMs are the largest class of Disinfection by-products (DBP) present in the most municipal waters. The four common THMs are Chloroform (CHCl3), dibromochloromethane (CHBr2Cl), dichlorobromomethane (CHCl2Br), and bromoform (CHBr3). Total Trihalomethanes (TTHM) is measured as the sum concentration of these four components. They are Cancer Group B Carcinogens (shown to cause cancer in laboratory animals). Chloroform is by far the most common in most water systems. Dibromochloromethane is the most serious cancer risk, followed in order by bromoform, chloroform and dichlorobromomethane. There have been studies that suggest a connection between TTHM and particularly bladder and possibly colon and rectal cancer. Table 1 indicates the Physical properties of THM Compounds.
TABLE 1PHYSICAL PROPERTIES OF THM COMPOUNDSMolecularComponentFormulaweightBoiling point, ° C.ChloroformCHCl3119.3861.0DibromochloromethaneCHBr2Cl208.29119.5DichlorobromomethaneCHCl2Br163.8387.0BromoformCHBr3252.75150.5
The requirement for high-purity water with particular properties has evolved in several industries. The water purity requirements of the semiconductor industry are among the most stringent of any industry. High-purity water processing procedures and the hardware required for carrying them out are complex and expensive. One such method is shown in U.S. Pat. No. 5,024,766, issued June 1991 to Mahmud, for point of use deionised water purification unit. The emphasis of this patent is on the reduction of organic contaminants. High-purity water treatment facilities adopt various techniques to achieve sub-ppb level of Total Organic Carbon (TOC) concentration. In the electronic Industry, organic compounds present in the rinse water can affect device yields and electrical characteristics. Organics have been implicated as causing “haze”. Irregular gate oxide densities were attributed to organic impurities in rinse waters. Current Ultra Pure Water standards for Electronics and Semiconductor Industry specify a TOC content of less than 1 ppb for deionised water that contacts the wafer surface during processing. It is important to note that the THMs are among the most difficult volatile organic component in TOC to treat.
Trihalomethanes are classified under Volatile Organic Compounds (VOC). VOC separation, especially removal at trace level, poses a challenge to the electronic industry where such impurities cannot be tolerated. Apparatus for the continuous removal of volatile organic halogenated compounds are known, for example, from U.S. Pat. Nos. 4,892,664; 5,004,484; 5,470,478; and 5,490,941.
The concentration of THMs and other halogenated organics in water can be reduced by adsorption with high iodine value GAC Filters to a limited extent. Their low molecular weight allows free passage through Reverse Osmosis (RO) membranes. UV 185 nm technology, commonly used for the reduction of TOC in high-purity water systems, also cannot modify chloroform to yield an ionic entity capable of ion-exchange removal. The THMs containing bromine are decomposed by the action of UV Radiation. Their boiling points are close enough to that of water to render them difficult to separate from water by distillation, particularly at trace levels. Vacuum degassification is promising for the lighter THMs, those with fewer bromine atoms, but less encouraging for their heavier analogues.
In PCT/US00/21422/(WO 01/12559) (The Coca-Cola Company) discloses a consolidated approach to water treatment (ie for removing microbiological contamination, bicarbonate hardness, VOCs/THMs, chlorine, turbidity and heavy metals) reliant on heating to breakdown bicarbonate hardness in the water to filterable precipitates and to issue gases from the water for collection in a headspace beyond a water contacting hermetic barrier. In this instance the water can pass through the membrane so gases are not selectively removed by the membrane (no membrane separation of any gas is involved).
Vacuum Degassification
A method for removing THMs and dissolved oxygen by vacuum degassing is shown in U.S. Pat. No. 6,277,175, issued August 2001 to Halder et al. The use of membrane contactors for degassing of high-purity water is known as disclosed in, for example, U.S. Pat. Nos. 5,264,171; 5,352,361; 5,938,922; and 6,402,818; Japanese Patent Nos. 2,725,311; 2,743,419; and 2,949,732; and commercially available products traded under the name of Separel. RTM hollow fibre membrane degassing modules from Dainippon Ink and Chemicals, Inc. of Tokyo, Japan, and Liqui-Cel. RTM membrane contactors from Celgard Inc. of Charlotte, N.C. However, a membrane contactor provides very little removal of THMs under conventional operating conditions.